Gas lasers typically contain a laser gas contained in a laser chamber. Normally two windows are provided for a laser beam to pass into and out of the chamber. These windows may be positioned normal to the laser axis or they may be positioned at various angles with respect to the beam axis. If the mirrors are normal to the laser beam axis the beam will suffer about 4% reflection loss at each window-gas interface and reflection from windows in the normal position may be detrimental to the laser gain and energy stability. By tilting the windows, even slightly, reflection caused problems can be reduced. By tilting the windows to the Brewster's angle (typically about 57°), the windows will have virtually 100% transmission for light whose electric component is parallel to the plane of incidence (defined by the beam axis and a normal to the window surface at the intersection of the beam axis and the window surface) see FIG. 1 at 7A and 7B. At angles smaller or larger than the Brewster's angle transmission of the parallel component is reduced by an amount dependent on the amount of deviation from the Brewster's angle.
As shown in FIG. 1, having both windows tilted at angles parallel to each other produces an offset due to the lateral displacement of the beam in the same direction in both windows. This complicates the alignment of the front and rear laser optics especially for large gas discharge lasers laser systems which typically are assembled from separate modules which are mounted separately on a laser frame. Therefore, in prior art gas discharge lasers with tilted windows, the windows are tilted in opposite directions as shown in FIG. 2 at 7C and 7D. This produces a “trapezoidal” shaped beam path. This arrangement of the mirrors also makes inspection of the windows for damage relatively easy since both windows can be viewed from the front of the chamber where doors to the laser cabinet are typically located. FIG. 2 describes a prior art excimer laser system used as the light source for integrated circuit fabrication. In this system, the laser chamber, the rear optic and the front optic are each separate modules mounted separately on the frame of the laser system.
The rear optic is a line narrowing module (typically called a line narrowing package or LNP) 2 and the front optic is an output coupler module 4. The laser chamber 6 can be removed from the front of the laser system and replaced or realigned without disturbing either the front or the rear optics and without significantly affecting the optical alignment. This is because the offset caused by one of the windows is cancelled by the other window. The LNP includes a three-prism beam expander 8, a tuning mirror 10 and a grating 12 positioned in a Littrow configuration.
FIG. 3 is a cross-sectional sketch of laser chamber 6 shown in FIG. 2. The chamber in addition to the laser gas contains elongated cathode 36A, elongated anode 36B, preionizer tube 46, insulator 42, anode support bar 35, heat exchanger 40 and tangential fan 38 for circulating the laser gas fast enough to clear discharge region 34 between successive pulses. For a 2,000 Hz pulse rate, this requires a gas velocity between the electrodes of about 30 m/s (about 67 miles/hour).
In this prior art laser system which typically operates at pulse repetition rates of 1000 to 2000 Hz, both the pulse energy and the wavelength are controlled with feedback control systems in which each laser pulse is monitored by power and wave meter 14 and the measured values are used by controller 16 to control the energy and wavelength of subsequent pulses based on measured values of pulse power and wavelength. Pulse energy control is achieved by controlling the charge on a charging capacitor bank in pulse power system 18 and the wavelengths of subsequent pulses are controlled by automatic adjustment of tuning mirror 10 by adjusting a drive arm of drive motor 20 to pivot the mirror. In this prior art laser system a change in the angle of illumination on the grating of 1.0 milliradian will result in a change in the selected wavelength of about 39 pm. A change in the direction of the beam exiting the chamber will also change the selected wavelength but because of the 26× prism beam expander the effect is a factor of 26 less. Therefore, a 1.0 milliradian change in the direction of the beam exiting the chamber will cause only a 1.5 pm. change in the selected wavelength.
As is evident from FIG. 2, the laser gas in the beam path within the chamber has the shape of a trapezoid which like a prism causes a very slight bending of the laser beam. For an ArF excimer laser with trapezoidal 45° mirrors with a three atmosphere mixture of 96.5% neon, 3.4% argon and 0.1% fluorine, the bending angle is 366 microradian as compared to a complete vacuum in the chamber. For the ArF laser shown in FIG. 2, a bending of the laser beam of 366 microradians corresponds to a change in the selected wavelength of 0.54 pm. This change is very small compared to the tuning range of tuning mirror 10, so that the bending of the beam caused by the gas “prism” is automatically compensated for by the wavelength feedback control. Also, a small quickly occurring change in the gas pressure during operation produces a very slight change in the wavelength which might be too fast for correction by normal feedback control. For example, a 2% change in the pressure at the same gas temperature would produce a wavelength change of about 0.011 pm.
In the past, operational variations in wavelength for these types of lasers have typically been in the range of about ±0.3 pm., and laser specifications on wavelength stability have been about ±0.5 pm. Therefore, in the past wavelength fluctuations (in the range of about 0.011 pm.) caused by small laser gas pressure changes in the trapezoidal shaped contained laser gas has not been recognized as a problem. Furthermore, as indicated above, if the pressure change is long compared to the wavelength feedback control cycle (which typically has been less than about seven milliseconds) any wavelength deviation from a target wavelength would be quickly and automatically corrected.
A known technique for measuring changes in light direction is to focus the beam on a spot partially blocked by a knife edge and to monitor the intensity of light not blocked. Changes in the intensity is a measure of the beam fluctuation.
Efforts have been made recently by Applicants and others to reduce wavelength variations and specifications on wavelength stability have become tighter.
What is needed in laser equipment and techniques to improve wavelength stability.